Heat dissipating plate and shell structures having desirable combinations of thermal conductivity, thermal expansion, and stiffness are in great demand for critical electronic applications. Good thermal conductivity is required to permit heat transfer away from high density electronic components and devices such as integrated circuits. The heat generated in electronic components causes high temperatures and requires, in many cases, that heat sinks be used to remove the heat from the electronic element or component. When heat sinks are not used, various other devices have been designed for direct attachment to a heat dissipating plate which, in turn, will act as the heat transfer element.
Moreover, the rapidly increasing density of integrated circuits, large-scale integrated circuits, power diodes, and the like has prompted growing requirements to satisfy more and more severe thermal conditions. Consequently, heat dissipating structures have undergone continuous improvements as the electronic industry has developed.
When the electronic components are attached to a heat dissipating structure, even one of the more advanced structures available today to provide good thermal conductivity, the different coefficients of thermal expansion of the various elements often cause problems. For example, the heat sink and the electrically insulating substrate, to which the electronic component is attached, may have different coefficients of thermal expansion. Similarly, different coefficients may exist between the electronic component and the heat sink.
When there is a significant difference in coefficients of thermal expansion between components, temperature changes arising from soldering and heat generated in the systems during operation can cause large thermal stresses due to the differences in relative growth of the materials. Such stresses may cause, in turn, premature component failure leading to reduced component reliability.
Metals such as aluminum and copper are most frequently used as heat dissipating structures because they have good thermal conductivity. The coefficients of thermal expansion of such materials are so high, however, that heat or cold will cause the premature failure of the electronic elements--an electronic element is usually made from a material with a lower coefficient of thermal expansion.
Efforts to produce a low expansion alloy, such as Invar, have led to materials with low thermal expansions, but which sacrifice thermal conductivity and density.
Semiconductors and other critical elements used in electrical components, circuits, or systems, such as silicon and gallium arsenide, are brittle, have low coefficients of thermal expansion, and generate considerable waste heat in operation. Consequently, the minimum requirements for heat transfer devices for these components in electronic systems are a low coefficient of thermal expansion and a high thermal conductivity.
No single monolithic metal has a low coefficient of thermal expansion and a high thermal conductivity. Composite materials made from various substrates and organic polymer materials, such as laminates made of a paper substrate and a phenolic resin or a fiber substrate of glass and epoxy resin, as well as ceramic materials such as alumina plates, have been used as substrates for printed wiring boards and heat sinks. The prior art suggests, however, that such substrate materials are defective because they have low thermal conductivity.
Other laminates have been developed which include a metal base and an organic polymeric material or metal-ceramic composite plates, such as an electrical insulating alumilite film formed on an aluminum plate. These laminates are inadequate, however, either because the organic polymeric material causes thermal resistance or because the alumilite film is subject to cracking. Moreover, the foregoing materials have coefficients of thermal expansion which are incompatible with that of the plate.
High conductivity structural materials have been created by placing high conductivity fibers (or particles) in a binder or matrix material. Plates of high conductivity and controlled thermal expansion are achieved by placing the fibers in preferred directions. The oriented fibers yield, however, anisotropic properties unless they are placed in several directions. In more conventional cases, combinations of layers of different materials are used to provide both good conductivity and controlled thermal expansion. However, panel thermal conductivities are often sacrificed to yield desirable thermal expansion properties in many cases.
U.S. Pat. No. 4,888,247 (Zweben et al.) discloses a specific, laminated heat conducting device. At least one layer of metal (aluminum) is bonded to at least one layer of polymer matrix composite material (epoxy resin). A low thermal expansion reinforcing material (graphite fibers) is distributed throughout and embedded in the polymer matrix composite material. In one embodiment, a plurality of alternating layers of metal and reinforced polymer matrix composite material form the device. The coefficient of thermal expansion and the thermal conductivity of the device are defined by the metal in the laminate in combination with the polymer matrix material and the reinforcing material within the polymer matrix material.
Thus, Zweben et al. focus on fiber reinforced resin matrix materials with layers of metal. The purpose of the combination is to use the metal for its high conductivity properties and the composite layers to restrain the thermal expansion of the metal so that the expansion of the composite is compatible with adjacent materials. As is typical of attempts to obtain a combination of properties by forming a composite of laminated layers, each material or layer in a plane interacts fully with the layers around it. Zweben et al. specifically disclose that the metal layer is bonded to the polymer matrix composite material. Such interaction requires a compromise in conductivity properties in order to achieve desired structural properties.
To overcome the shortcomings of existing low-thermal expansion, high-thermal conductivity structures, a new hybrid construction is provided. One object of the present invention is to provide a structure which has a high thermal conductivity for transferring heat generated by electronic components and systems.
A match between the coefficient of thermal expansion of adjacent components in electronic devices is critical because it prevents structural and electrical failure during thermal cycling over the operational range of the components. Accordingly, another object of the present invention is to provide an improved structure which can be tailored to match the coefficient of thermal expansion of adjacent elements.
A structure which combines both high conductivity and a low coefficient of thermal expansion which can be tailored to provide desirable mechanical properties (e.g., strength and stiffness) is a further object. It is still another object of the present invention to provide a structure which has controlled properties in different directions, including in-plane isotropic properties as one special case.